This invention relates to spread-spectrum communications, and more particularly to code-division-multiple-access (CDMA) cellular, packet-switched systems. The inventive concepts involve use of a start of message indicator as a collision avoidance mechanism, to reduce the impact of remote terminal mistakes in perceived successful channel access attempts.
Recent developments in wireless communications technologies have allowed expansion of service offerings from the original voice telephone service model to include a number of services supporting packet data communications. As customers become increasingly familiar with data services offered through landline networks, they are increasingly demanding comparable data communications in the wireless domain, for example to maintain service while mobile subscribers roam freely or to provide remote service in locations where wireless loops are preferable to landline subscriber loops. A number of technologies that support packet data communications in the wireless domain utilize code division multiple access (CDMA), which involves direct sequence spread-spectrum encoding. In such systems, different physical channels are defined by use of different channelization and/or spreading codes, as part of the direct sequence modulation process.
A CDMA-based random access channel (RACH) provides uplink packet transport from a mobile station (MS) to a base station (BS), with a random slotted-ALOHA type procedure to access the channel resources. U.S. Pat. No. 6,169,759 to Kanterakis et al. discloses a common-packet channel (CPCH), which provides a similar uplink transport for transmitting variable size packets from a mobile station (MS) to a base station (BS). The disclosure of U.S. Pat. No. 6,169,759 to Kanterakis et al. is entirely incorporated herein by reference.
The RACH and CPCH channels do not need direct resource allocation. The channel resource allocation of these channels is contention based. The mobile station transmits an access preamble corresponding to a channel that the mobile station desires to use. The base station responds with a matching preamble that signals successful access to the selected channel resource.
FIG. 1 is a simplified example of the signals exchanged between a mobile station and a base station for a CPCH service. The mobile station selects one of the available uplink channels through a base station. The access phase involves the MS-spread-spectrum transmitter sending one or more access preambles (AP) over an uplink physical channel, in access slots defined in relation to a frame-timing signal derived from receipt of the common synchronization channel (not shown). The access preamble (AP) contains a signature corresponding to the selected uplink channel, that is to say the one channel that the mobile station is attempting to access from among those available by the base station. When the base station receives an access preamble (AP) at an adequately detectable power level, it transmits back an acknowledgement (ACK), containing a signature that corresponds to the access preamble signature, over the indicator channel.
The mobile station ceases transmission of the access preamble (AP) when it receives the corresponding ACK signal from the base station. If the mobile station successfully receives the acknowledgement corresponding to the access preamble that the station transmitted, the mobile station proceeds to the next phase in the transmission process, shown generally as transmission of data and control information over the uplink channel in FIG. 1. Alternatively, the mobile station will cease its transmission of access preambles if the mobile station has transmitted the maximum allowed number of access preambles. In this later situation, the mobile station assumes that its access attempt has failed, so the station backs off and waits for some period of time before initiating another access attempt.
As shown in FIG. 2, the message part carried over the downlink physical control channel (DPCCH) is divided into 10 msec frames. Each 10 msec frame is split into 15 slots (0, 1, 2, . . . 14), each of length Tslot=2,560 chips. As shown, each slot carries 10 bits of information. Each 10-bit slot of the downlink control channel contains fields comprising Pilot, CCC, TFCI and TPC. The TPC field carries transmission power control (TPC) bits. The TFCI field carries the transport format combination indicator, and the CCC filed carries four data bits for CPCH control command information.
Although these CDMA-based communication technologies offer enhanced packet data communications, problems still arise that cause collisions. It has been found that there is roughly a 30% chance that two or more preambles from mobile stations will arrive at the base station in any given 50 ms time-window. One mobile station may then mistake a subsequent acknowledgement signal intended for another mobile station to be one intended for itself. The mistaken mobile station will then transmit its packet on a channel intended for use by another mobile station. This will lead to excessive interference. Moreover, if the mistaken mobile station sends its packet over an already busy channel, there is a great chance of a cell-shut-down. This is extremely undesirable in cellular telephony as users in the neighboring cells will also be adversely affected.
Various methods of collision detection, collision resolution and channel assignment were developed to reduce the occurrences of these collision. A CPCH system, such as that disclosed by Kanterakis et al., utilizes a collision detection phase at the start of the data and control communications over the uplink transportation channel and the downlink control channel, to allocate the uplink channel to a mobile station that successfully avoids collision. If two or more mobile stations are still attempting access to the same channel at the time of the collision detection phase, the base station may respond with at most one matching collision detection preamble, effectively allocating the channel to one mobile station. In some cases, the base station will not be able to resolve the collision detection and will not send back any collision detection preamble. A mobile station that fails to receive its matching collision detection preamble from the base station aborts its access attempt.
The collision detection approach does reduce collision problems. However, none of these methods can totally eliminate the chance of collision due to unintended errors. For example, a mobile station may misinterpret its channel assignment and proceed to transmit in a channel other than the one it was assigned, even though the other channel may be assigned to and in use by a different mobile station. There are several situations that may lead to such a mistake. For example, in CPCH, two mobile stations, MSA and MSB, have sent in AP1 and AP2, respectively. The base station has only responded with an AP ACK1, which corresponds to AP1. MSA has correctly identified the ACK, however, MSB has mistaken the ACK to correspond to AP2. The mistaken mobile station may then proceed with uplink transmission over the channel corresponding to AP2, which may already be in use by another station.
Although the above-described collision detection procedure reduces the probability of such collisions between stations attempting access on the same channel at about the same time, mistakes regarding attempted accesses to different channels still can occur as a result of substantially concurrent collision detection phases for different channels. In this situation, assume that mobile station MSB is waiting for a collision detection acknowledgement. At about this time, the base station sends a CD acknowledgement for the mobile station MSA that was attempting to access another channel. The one mobile station MSB, however, mistakes the CD acknowledgement as an acknowledgement of its own CD preamble and begins transmitting over the channel that it is attempting to access, even though that channel may already be in use by another station.
As noted, when a mistaken mobile station starts to transmit a message, it creates excessive interference and may even lead to a cell shut down. Two mobile stations transmitting over the same channel can create instability in the wireless system, because mobile stations need to be closely controlled with regard to their transmit power in any CDMA system. If a mobile station MSB assumes that it is being power controlled, when in fact it is not, it is possible that the mistaken mobile station will increase its power to an unacceptable level. In the case of a mistaken channel assignment, the power control problem can happen because the mobile station MSB is mistakenly transmitting on a channel assigned to another mobile station MSA and any power control information on the corresponding downlink channel is actually intended for a different mobile station. The base station may not be detecting the transmissions of the mistaken mobile station MSB, at all.
As shown by this discussion, there is a need for a technique to avoid collisions in a spread-spectrum packet communication system, which will mitigate the above noted problems, particularly those due to mistaken interpretation of channel assignment by contending mobile stations.
Hence a general objective of the invention is to reduce the occurrence of colliding transmission of packets within a given uplink physical channel of a wireless packet data communication system, utilizing spread spectrum transmissions.
Another objective of the invention is to provide a technique that will enable a mobile station in such a system to recognize when it may be transmitting on a channel that has not actually been assigned to that mobile station, so that the mistaken station will promptly cease such transmissions.
One aspect of the current invention relates to methods, which can be used as collision resolution mechanisms alone or in combination with other methods commonly known in the art. The methods provide a start of message indicator or other known sequence on the downlink channel. These methods involve transmission of the known sequence at the beginning of transmissions, for example as a start of message indicator, over a downstream channel corresponding to the desired uplink channel. Use of this sequence serves to eliminate mistaken transmission over an intended uplink channel by a mobile station that has misinterpreted access-related signals regarding another channel as an acknowledgment or the like with regard to its accessing of the intended channel.
Accordingly, a first aspect of the present invention, as embodied and broadly described herein, relates to an improvement to operations in a CDMA system which supports packet-switched communication. The CDMA system has a plurality of base stations (BS) and a plurality of mobile stations (MS). For packet-switched based communication, there are also a plurality of uplink transport channels and a plurality of downlink control channels corresponding to each of the plurality of uplink transport channels. The inventive method comprises the steps of one or more mobile stations attempting to establish a link with a base station. At a mobile station, upon a successful or perceived-to-be-successful attempt, the steps include beginning its data transmission via a first uplink transport channel. The selection of the first uplink transport channel can be based on a mobile station self-selection, a base station channel assignment, or other mechanisms commonly known in the art.
At the base station, the steps include receiving attempts from one or more mobile stations to establish links with the base station and selecting an intended mobile station to transmit its data over an intended uplink transport channel. The selection can be on a first-come-first-served basis, on a random basis, or be based on performance factors of the one or more mobile stations. The steps further include transmitting a sequence known to the intended mobile station during the first frame of the data transmission for the intended mobile station over a downlink control channel corresponding to the intended uplink transport channel. The sequence can be repeated for several frames to increase reliability.
When the mobile station begins its data transmission via the first uplink transport channel, it also listens for the known sequence to the transmitted over the downlink control channel corresponding to the first uplink transport channel. Upon successful detection of the known sequence by the mobile station, the steps include continuing transmission of its data over the first uplink transport channel. However, upon no detection of the known sequence by the mobile station, for example within a predetermined interval following the start of data transmission, the mobile station shuts off its uplink data transmission immediately.
The inventive collision avoidance technique admits of a wide range of variations and applications. For example, the preferred embodiments involve application to CDMA type wireless communications, particularly for CPCH-based packet data services. However, the invention may find application to packet data communications in other types of digital wireless networks. Other aspects of the invention relate to implementations of base stations and mobile stations, which take advantage of the inventive collision avoidance techniques.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following and the accompanying drawings or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.